Two pairs of miniaturized platinum and reference electrodes are set in a cap which tightly fits an optical cuvette. The oxidizing potential of a solution or suspension of respiratory components is measured and used to direct pulses of electricity through the second pair of electrodes, from a constant current generator, in order to change or maintain the oxidizing potential of the cuvette contents. The cuvette is placed in a split-beam dual-wavelength spectrophotometer so that the redox state of its cytochromes can be optically monitored. This experimental set-up is to be used for potentiometric titrations and for conducting and storing a series of spectral scans at fixed potentials. The entire system is integrated with and under the control of a microcomputer.